The present invention relates to nuclear imaging systems and more particularly to an apparatus for providing a point source of radiation to an annular positron emission tomography scanner for checking detector operation.
Positrons are positively charged electrons which are emitted by radio nuclides that have been prepared using a cyclotron or other device. The radio nuclides most often employed in diagnostic imaging are fluorine-18 (.sup.18 F), carbon-11 (.sup.11 C), nitrogen-13 (.sup.13 N), and oxygen-15 (.sup.15 O). Radio nuclides are employed as radioactive tracers called "radiopharmaceuticals" by incorporating them into substances such as glucose or carbon dioxide. One common use for radiopharmaceuticals is in the medical imaging field.
To use a radiopharmaceutical in imaging, the radiopharmaceutical is injected into a patient and accumulates in an organ, vessel or the like, which is to be imaged. It is known that specific radiopharmaceuticals become concentrated within certain organs or, in the case of a vessel, that specific radiopharmaceuticals will not be absorbed by a vessel wall. The process of concentrating often involves processes such as glucose metabolism, fatty acid metabolism and protein synthesis. Hereinafter, in the interest of simplifying this explanation, an organ to be imaged will be referred to generally as an "organ of interest" and prior art and the invention will be described with respect to a hypothetical organ of interest.
After the radiopharmaceutical becomes concentrated within an organ of interest and while the radio nuclides decay, the radio nuclides emit positrons. The positrons travel a very short distance before they encounter an electron and, when the positron encounters an electron, the positron is annihilated and converted into two photons, or gamma rays. This annihilation event is characterized by two features which are pertinent to medical imaging and particularly to medical imaging using photon emission tomography (PET). First, each gamma ray has an energy of essentially 511 keV upon annihilation. Second, the two gamma rays are directed in substantially opposite directions.
In PET imaging, if the general locations of annihilations can be identified in three dimensions, a three dimensional image of an organ of interest can be reconstructed for observation. To detect annihilation locations, a PET scanner is employed. An exemplary PET scanner includes a plurality of detectors and a processor which, among other things, includes coincidence detection circuitry. For the purposes of this explanation it will be assumed that a scanner includes 12,000 detectors which are arranged to form an annular gantry about an imaging area wherein the scanner thickness (i.e. parallel to an imaging axis which passes through the imaging area) is 6 detectors thick. Each time a 511 keV photon impacts a detector, the detector generates an electronic signal or pulse which is provided to the processor coincidence circuitry.
The coincidence circuitry identifies essentially simultaneous pulse pairs which correspond to detectors which are essentially on opposite sides of the imaging area. Thus, a simultaneous pulse pair indicates that an annihilation has occurred on a straight line between an associated pair of detectors. Over an acquisition period of a few minutes millions of annihilations are recorded, each annihilation associated with a unique detector pair. After an acquisition period, recorded annihilation data can be used via any of several different well known back projection procedures to construct the three dimensional image of the organ of interest.
As well known in the PET art the signals generated by a PET detector vary (e.g. typically become less intense) in intensity over the life of the detector and, indeed, at some point each detector reaches a point where the signal generated thereby is insufficient for imaging purposes. As a detectors signal changes, the signal must be compensated to account for the change. To this end, it is important to have some type of system which periodically test each scanner detector to determine detector condition. Once detector condition is determined detector compensation can be modified to compensate for any perceived degradation. In the event that detector condition has deteriorated to the point of being unusable, even when a signal generated thereby is compensated, the detector can be removed and replaced.
Generally, to test a detector, a point radiation source having a known energy level and intensity is placed adjacent a detector face thereby causing the detector to generate an intensity signal. The generated signal is then compared to a desired intensity signal which corresponds to the known energy level and intensity. If the generated signal is different than the desired signal the gain of the generated signal is modified to cause a resulting signal to be equal to the desired signal.
While the general requirements of a detector checking system are relatively simple, the industry has failed to come up with a simple apparatus to facilitate the requirements. For example, one solution has been to provide a tube which forms a tight spiral path, the internal surface of the tube forming a helical channel, the external surface of each tube winding contacting the external surface of an adjacent tube winding such that the tube as a whole defines a cylinder. The cylinder is dimensioned such that the cylinder fits within the imaging area adjacent the PET scanner. The external surface of the cylinder is radio-translucent so that radiation passes therethrough while the internal surface of the cylinder is covered with a radiation blocking material. A point source of radiation is provided inside the tube and some mechanism is provided to control the source position within the tube. For example, the tube may be provided with a liquid source and a pump which can control source position by pumping liquid into and out of the tube. Another system may include a metallic bead string having proximate and distal ends wherein the proximate end is linked to a winder for winding and unwinding the string and the distal end is linked to the point source. When the string is unwound the source is forced along the tube in a first direction and when the string is would the tube source is pulled within the tube in a second direction opposite the first.
In operation, to use the tube to assess detector accuracy, the cylinder is placed inside the imaging area so that the center of the tube is concentric with the imaging axis. The point source is placed within the tube and the movement mechanism (e.g. liquid or metallic string) is controlled to cause placement of the source adjacent detectors. Where the tube has a relatively minimal girth the source can be placed in front of any of the detectors to assess detector operation.
While these solutions for providing a detector source have proven useful, these solutions have a number of shortcomings. First, the tube configuration is relatively hardware intensive and therefore is also relatively expensive.
Second, the time required to position a source adjacent a specific detector is excessive considering the task to be accomplished. For example, where a detector to be examined is positioned axially distant from the proximate end of the tube, to examine the detector these types of systems would require a user to align the cylinder within the imaging area and then cause the source to pass through potentially the entire tube to reach the axially distant detector position. This is extremely time consuming. In addition, between detector examinations and during imaging sessions, the cylinder must be removed from the imaging area so that the detectors can be used for regular emission imaging. While cylinder removal can be relatively quick, cylinder replacement and alignment within the imaging area must be extremely accurate to ensure that specific sections of the tube are adjacent specific detectors. This alignment process can be time consuming and therefore acts as a barrier to frequent detector testing.
Third, there are some concerns about point source position accuracy with these types of systems. For example, if there is some slack in a metallic string or some liquid passes a point source within the tube, position accuracy can suffer.
Fourth, because tube girth has to be relatively minimal tube manufacturing tolerances are extremely tight which further increases the costs of tube-type systems.
Another solution to provide a source for testing detector accuracy has been to build a light source of known intensity into each detector. In this case, to determine accuracy of a specific detector the detector source is turned on and a detector signal is generated for comparison to a desired signal. While this solution is fast and accurate, detectors with built in sources are relatively expensive and therefore cost prohibitive. In addition, existing systems cannot usually be retrofitted to accommodate such detectors.